Chromium plating



Patented Mar. 2 4, 19 31 UNITED STATES.

PATENT o Fica JOHN F. K. mcunnouan AND BENJAMIN W. GILCERIST, OF DETROIT, MICHIGAN, AS-

SIGNORS TO TERNSTED'I MANUFACTURING COMPANY, OF DETROIT, MICHIGAN, A

, Ho Drawing.

CORPORATION OF MICHIGAN CHBQMIUM rnarmo Our discovery relates to chromium plating. The prevailing chromium plating baths are the chromic acid baths founded on the socalled Sargents solution, with certain modifications. The chromic acid bath, however, is

very de'ficientl in throwing capacity, and great difficult}; is experienced in properly chromium-plating irregular articles. This often requires special shaped-anodes, and

even then therecesses are often not plated at all,.or are very thinly plated.

In our prior applications, Serial No 198,689 filed}? June 13, 1927, and Serial No. 264,952 filed Mar. 26, 1928, we have described and claimed a process of/chromium plating,

, and a-bath which employs chromium chlorideprocesses.

the better the conductivity of the bath, but

We employ a bath of bichromate of soda, hydrochloric acid and oxalic acid. For example, we take 240 grams of bichrom'ate of soda dissolved in one-half liter of warm water; to this is added 154. grams concentrated hydrochloric acid; to this is added 3.75 grams of hydrated oxalic acid. We then addenough water to bring the bath up to one liter. The .bath is then completely stirred up. It will be obvious that for larger or smaller baths the proportions will be varied accordingly.

This bathis operated preferably anywhere from room temperature up to 160 degrees Fahrenheit, and the higher the temperature employs Application filed March 29, 1928. Serial No. 265,808. A

higher temperature lowersthe polarization of the cathode, and consequently lowers the throwing ability of the bath. This bath is preferably operated at a volta e of from 4 to 13 volts. The amperage of t is bath will be anywhere from 100 amperes to 500 amperes per square'foot of cathod'e. The amperage will vary substantially,directly as the voltage. v

The above bath is what We call a. rapid bath,that is,- the deposits are much faster than a hath made up'as 'given in the example .below, which we call a slow bath. Such a slow bath may be made up by dissolving 360- gram'sbichromate of]: sodain half a liter of warm water; to this is added 77 grams hydrochloric acid, and then 240 grams oxalic-acid.

Water is then added to make the bath up to one liter. The bath can be operated at room temperature, up to 160 degrees F. and With a voltage of from 4 to 13 and a corresponding amperage of 100 to 500 amperes per squarefoot of cathode. In this bath, however,

the deposit will be relatively slower than the I previous bath. This is due to a larger percentage of trivalent chromium, to wit: chromium chloride. There is a higher alkalinity at the cathode "than in the other bath, due to less hydrochloric acid.

WVe find that the above baths can be operated successfully without the use of oxalic acid,'alt-hough we much prefer to use the oxalic acid as it aids in reducing the hexavalent chromium to the trivalent form.

The bichromate of soda, Na Cr O +2HCl (hydrochloric acid) in solution, reacts to form 2CrO ClONa+H O, the first ingredient being chlorochromate of soda plus water.

' To this the oxalic acid is added, which reacts to form chromium chloride, which in turn reacts with the chlorochromate to form chromium chromate.

These reactions are: 2NaCrO Cl+3H O O +'6HO1= 2NaCl+2CrCl GCO +-6H O NaCl Sodium chloride (common salt) acetic acid, phosphoric acid, and others, as CrCl Chromium chloride the main urpose of the free acid is to pre- CO Carbon dioxide vent -alkal1nity at the cathode, and-also as a H O Water conductor.

NaCrO Cl Sodium chlorochromate What we clalm 1s:1 h 1 d Na'JC'rOBCL+2CrC13+3H2O= 1. A chromium p atmg at inc 11 mg NaCrO Cl Sodium chlorochromate w H Water CrCl Chromium chloride. XCr O YCrO Chromium chromate NaCl Sodium chloride (common salt) 15 HCl. Hydrochloric acid renders the From reading the above reactions it will be seen that carbon dioxide passes 03 as a gasv and chromium chloride is formed, and sodium chloride and water. The above reaction of course is in only part of the bath, and the chromium chloride thus formed in the last reaction then reacts with the chlorochromate forming chromium chromate (XCr OsYCrO The final constituents of the bath are therefore: chlorochr'omate of soda, chromium chromate, common salt, and water, and some.

excess hydrochloric acid that has not reacted with the other ingredients.

We find that a bath so constituted has great throwing power. The throwing power depends upon the polarization of the cathode, the conductivity of the bath, and the cathode efficiency. All of these are-high in a bath made in accordance with the above directions I and consequently the throwing power is very good.

anodes or insoluble graphite or magnetite anodes. Care must be taken in proportion- "ing the amount of chromium anodes to in- Bichromate of soda may be easily procured Furthermore, carbon dioxide is the.

when ready for operation, chlorochromate of soda, chromium chromate, hydrochloric acid, and water.

2. A chromium plating bath which when ready for operation includes an alkaline metal chlorochromate, and chromium chromate and an acid.

3. A chromium plating bath, which when ready for operation includes an alkaline metal chlorochromate, chromium chromate, and hydrochloric acid.

4. A. chromium plating bath which when ready for operation includes one of the alkaline metal or earth metal chlorochromates.

5.. A chromium plating bath which is made up solely of bichromate of soda, hydrochloric acid, water and a reducing agent.

6. A chromium plating bath which 1s made up solely of bichromate of soda, hydrochloric acid, oxalic acid and water.

7. A chromium plating bath which is made up solely of an alkaline metal or earth metal bichromate, hydrochloric acid, and a reducing agent. a

In testimony whereof we have aflixed our signatures.

. JOHN F. K. MoCULLOUGH. BENJAMIN W. GILGHRIST.

soluble anodes, due "to the fact'that'the chromium anode dissolves in three valencies and thetrivalent chromium would thereby lower the efiiciency of the solution.

The bichromate of soda may be replaced by the chromates or bichromates of the alkaline metal or alkaline earth metals, such as ammonium and potassium of the alkaline metal group, and barium, strontium, and cal-' cium, of the alkaline earth metal group.

The free hydrochloric acid in the bath may be replaced by numerous other acids, such as 

